Editor’s Note: This article is the
second of two that addresses the development of the human brain. Last month’s
article, “The Evolution of Human Capabilities and Abilities,” focused on
neurons, the basic information-processing units of the nervous system. This
month’s article examines the evolution of the neocortex, a part
of the cerebral cortex concerned with sight and hearing in mammals, regarded as
the most developed part of the cortex.

Source/Shutterstock

Compared
to other mammals, one cannot help but be impressed with the abilities of humans
to imagine things, ponder the past and future, communicate through speech and writing,
and understand the mental states of others. Humans are also capable of a deep
understanding of the physical properties of materials and objects that can be
used as tools or for building; they have the ability to recognize and know
countless other people, and to interact and cooperate for the greater good. We
are culturally adaptive to the extent that we occupy most of the ecosystems on
earth. And if that’s not impressive enough, we are planning to inhabit a space
station on Mars.

Individual humans are highly variable in what
they know and what they can do, as evidenced by athletes, musicians, scholars,
writers, and architects. The best business leaders probably do not know how to
farm and vice versa. All these varied and profound abilities depend on our
brains—and especially our expansive neocortex.

Our understanding of how our brains execute
these impressive abilities is fragmentary. Nevertheless, we are rapidly learning the ways in which brains vary
across the classes (taxa) of mammals, and how these differences allow for human
abilities.

The
distinctive features of human
brains emerged and further
differentiated as our ancestors evolved, especially over the last two million
years, with the modern human brain reaching its current potential perhaps as
long as 200,000 years ago. Here, we consider brain features that are enhanced
compared to those of other mammals, and likely account for our unique
abilities.

Size Matters

Obviously, having a larger brain must present
some advantages to compensate for its huge metabolic cost and long developmental time. As brain size is relatively easy to measure,
there is a long history of research relating brain size to cognitive and other
mental abilities,1 and meaningful correlations
have been produced repeatedly.2

The range of brain sizes across mammals is
huge, ranging from the tiny shrew (0.06-3.0-grams) to the African elephant(4,600-grams).2 As a general rule, mammals with small brains have limited behavioral abilities,
while those with large brains have greater cognitive and mental capacities.

The spectrum across primate species, while less than that of all mammals,
is also significant, ranging from the smallest primate brain of the appropriately named mouse
lemur—roughly 1.5 grams—to the
largest of primate brains, the human brain of 1,200 to 1,400 grams or more. The mouse lemur is well adapted to varied
habitats in Madagascar, but its behaviors resemble those of the house mouse
more than those of the human.

A more useful comparison may be with
chimpanzees, our closest relatives, whose brains of around 340 grams are
roughly one-quarter the size of ours. Chimpanzees are clearly very intelligent
and have visual and other sensory abilities that match or perhaps exceed those
of humans. Although they cannot produce speech, some chimpanzees can learn the
meanings of symbols for objects and acquire an understanding of hundreds of
English words. They are, however, severely limited in the cognitive abilities
involved in using tools or instruments to retrieve and acquire food. Compared
to humans, they are also limited in their ability to infer or understand what
others know.

But there are reasons to be cautious about
assuming that brain size is the whole story. Brain size correlates with body
size; oftentimes even within a species. As body size is determined by both
genetic and environmental factors, body and brain sizes tend to be smaller in
females of our species, and in members of cultures less exposed to modern fast
foods. In the past, this led scholars to
underestimate the mental abilities of women and
individuals reared on fewer calories.

In addition, some mammals (especially
primates) and birds do better than expected for their brain sizes. The question
is why the relationship between brain size and behavior generally holds—and
when it doesn’t, why not?

Neurons vs. the Neocortex

A modern understanding of the number of
neurons in brains, or parts of brains, of various vertebrates comes from the
recent studies of Suzana Herculano-Houzel (author of last month’s Cerebrum article) and her research team.4 If larger brains have more
computational capacity, and the basic computational unit of the brain is the
neuron, the total number of neurons in the brain should correlate better than
brain size with brain functions, especially if brain size does not fully
predict numbers of neurons.

In a series of important studies,
Herculano-Houzel determined the number of neurons in the 19 orders or taxa of mammals
with brains of different sizes. While her research focuses on neurons, we will
focus on a part of the brain called the forebrain, and especially the neocortex,
which is the structure that is most important in cognition. For most orders of
mammals, increased neocortical size is not matched by an equivalent increase in
neuron number: a doubling of cortex leads to something less than a doubling of
neurons. Instead, neurons tend to get bigger as the cortex gets bigger, except in primates.

In primates, the relation of brain size to
numbers of neurons is constant, i.e. the scaling is isometric. This is much
like Alice in Alice in Wonderland: as
Alice shrinks to a much smaller size or grows much larger, the proportions
among her parts remain the same. In primates, this is the case for the brain in
general and for the neocortex in particular, as increased brain size is mainly
in the cortex. As cortex doubles in size, the number of neurons doubles. In
humans, the cortex makes up more than 80 percent of the brain mass, and has
roughly 16 billion neurons, more than any other mammal, including elephants and
cetaceans with much larger brains.

The great number of cortical neurons in human
brains reflects the exceptional cognitive abilities of humans. However, as
Herculano-Houzel points out, this greater number of neurons is exactly what
would be predicted from primate brain scaling laws. Although the human cortex
has the number of neurons predicted from cortical size for primates, the result
makes humans unique among mammals overall by having more neurons in cortex than
non-primates with even larger amounts of cortex. This is important because the
number of neurons in the cerebral cortex correlates better with cognitive
abilities than with absolute cortical size or cortical size relative to body size.5

The studies of neuron numbers in brains, and
parts of brains, reveal two other important points. First, birds are very
successful vertebrates. Their brains are small, yet they appear to have high
levels of cognitive abilities. Within the nearly 11,000 species of birds,
cognitive abilities are especially high in the Corvidae family of birds (jays,
ravens, crows, and magpies) and parrots. These species also have relatively
larger forebrains than
other birds. In addition, all birds have forebrains that are more densely packed with more small neurons
than the forebrains of mammals, even primates
with somewhat larger brains.6 Thus, for both primates and birds,
neuron number better predicts cognitive abilities than forebrain size.6

Second, the cerebellum is a part of the brain that contains most of
the brain’s neurons.4 While the cerebellum has been implicated
in cognitive functions, it is predominantly concerned with motor control, which
occurs without conscious effort and depends on huge numbers of very small
neurons. Because neurons are so densely packed, the size of the cerebellum does
not keep pace as the neocortex gets larger. However, the numbers of neurons in
the two regions tend to remain proportional.

This relationship suggests that the specialized
functions of the neocortex and cerebellum are aligned in some manner, but that
cognitive functions are more closely tied to the former rather than the latter.
The elephant brain, three times larger than the human, has nearly 98 percent of
its neurons in its cerebellum, but only one third the number of cortical
neurons as humans. This disproportion indicates that the relation of
cortical-to-cerebellar neurons is not necessarily fixed,4 and
supports the above conclusion about the relative importance of the two areas in
cognition.

But the number of neurons in the cortex does
not fully explain human abilities, either. For example, why did Neanderthals,
with brains as big as ours and as many neurons, become extinct while we have
populated the earth? And how can human children treated for epilepsy by having
much of the right cerebral hemisphere removed, grow up to be highly functional
adults?

Cortical Structure

The neocortex of mammals is subdivided into
functionally specialized zones. KorbinianBrodmann, a Germanneurologist in the early 20th
Century who became famous for parsing the cerebral cortex into 52 histologically
distinct regions (known as Brodmann areas),
considered these areas to be functional equivalents of the organs of the body,
such as heart, lung, and kidney.7 While this definition has
proven useful, it has beendifficult to
identify and delimit such cortical areas.

Brodmann characterized the cortical landscape
on the assumption that differences in regional function are reflected by
differences in structural specialization. He accordingly based his depictions
of brain areas on variations of neurons within structures. Today we more
reliably divide the cortex into areas of likely functional significance with an
array of staining procedures that reflect both structural and functional
differences among neurons.8 In addition, we define sensory and motor
areas using functional maps of sensory inputs or motor outputs, patterns of
cortical and subcortical connections, and imaging of cortical activation
patterns. Agreement across such methods provides the strongest evidence for a
specific area’s existence.

The number of cortical areas clearly varies
across mammals, increasing with brain and neocortex size. Those with small
brains and little neocortex may have 20 or fewer.9 Primates may
range from 50 areas or less in those with smaller brains and less neocortex, to
200 or more per cerebral hemisphere in humans.9,10

We know much less about the cortex of other
large-brained mammals such as whales and elephants, but cortical architecture
suggests that they have far fewer cortical areas than humans. The human brain,
nearly uniquely, adds to the total number of its functional areas by
differently specializing corresponding parts of each hemisphere. We have areas
for language, for example, in the left cerebral hemisphere, and areas for
visuospatial and attentional abilities in the right.10

Humans also have or share a sub-areal
organization in some areas of the cortex that increases processing capacity.11
As an example, primates divide primary visual cortex, V1, into two sets of cortical
columns of neurons, one of which processes information about color and
contrast, and the other selective for specific orientations of edges in visual
stimuli.

More structural and functional divisions of
cortical layers in primates, especially in humans, may offer more ways of
increasing processing capacity. The six primary layers of cortex in all
mammalscreate functionally distinct
channels. In primates, sublayers within are further specialized. And a
subdivision of one of these layers in primary visual
cortex appears
to be specialized differently in humans than in other primates.13

Overall, humans probably exceed all other
mammals in numbers of cortical areas. This creates more and longer processing
streams, and more interactions between streams. Sensory information and
memories can be evaluated and processed more extensively, and in more
different ways, in human brains. As cognitive psychologist Steve Pinker has
emphasized, the neural computations at each processing step may be rather
simple, but a series of these steps can result in complex outcomes.14

Human Uniqueness

In humans and other mammals, neurons vary
structurally and also in the neurotransmitters and receptors they use to
communicate with one another. Such features allow neurons to function
differently and promote specialization.15

Neurons may be more varied in humans than in
other primates and most other mammals. Structurally, our most common type of
neuron is the pyramidal cell, which varies considerably inthe size of the cell body and of its dendritic
arbor (which receives electrochemical messages from neighboring neurons). A
large cell body reflects the need to sustain a long, thick axon (to transmit a
message), or a widespread dendritic arbor, or both. A small cell body is
usually paired with a small dendritic arbor and a short, thinner axon. Neurons with
large arbors usually receive small numbers of synaptic inputs from many sources
and integrate these inputs. Those with small arbors usually are powerfully
activated by just a few input axons and are good at preserving information for
distribution and further use.

The smallest neurons, located in sensory areas
of some primates, respond to only a few axons from neurons in the brain’s
thalamus, and then relay those messages only to nearby neurons. Neurons that
transmit inhibitory messages, which constitute 20 to 25 percent of all cortical
neurons, are typically also small.

One class of inhibitory neurons, the double
bouquet cell, is found in the cortex of primates but not in other mammals. Von
Economo neurons, large, spindle-shaped cells with a simple dendritic arbor,
were thought to be present in only a few cortical areas in only humans and
certain apes, but have more recently been observed in other mammals with large
brains. Overall, primate brains appear to have more specialized and unusual
neuron types, and this variety may contribute to brain function, especially in
humans.

Some cortical areas are noteworthy in having
neurons with specialized features. Thus, primary sensory areas often have a
thick layer that is densely packed with small neurons. In monkeys, apes, and
humans, these neurons are especially small. In addition, the pyramidal neurons
of these primates are smaller than in other cortical areas. Overall, neurons in
this area are packed three-to-four times more tightly than in most of the rest
of the cortex.16

Thus, the large primary visual area of
primates, especially humans, is well designed to preserve and distribute
details of the visual image. Yet primary visual cortex has some neurons
specialized to integrate information. A scattering of very large pyramidal
neurons, known as Meynert cells, have widespread basal dendrites that gather
information over a large expanse of cortex. They send information related to
visual motion and change over thick, long, rapidly conducting axons to more
distant parts of the brain.

The primary motor cortex in humans and other
primates is also characterized by very large pyramidal neurons in layer five,
which are especially large in humans. These Betz cells summarize a lot of
information and activate motor neurons in the brainstem and spinal cord over
thick, long axons. Other pyramidal neurons in the primary motor cortex (M1) are
also large and summarize many inputs, while the small neurons are almost
totally missing. M1, especially in humans, is thus specialized for summing
information from many sources, as the final cortical location for producing
actions.

Primates also have a unique part of the
prefrontal cortex, a dorsolateral region known as the granular prefrontal
cortex.17 As the name suggests, it is distinguished by a
layer of small granular neurons, a feature that would preserve input
information. Yet, it also has large pyramidal neurons in another layer,
implying an integratingfunction. Thus,
this region of the cortex, unique in primates and expanded in humans, appears
to have both information-preserving and summing functions in different layers
of cortex, much like the primary visual cortex.

Human Benefits

Much of motor performance is controlled by
subcortical centers in the brainstem and spinal cord. Early mammals got along
fine without primary motor and premotor cortical areas, as do present-day
monotremes and marsupials. For early mammals, cortical motor control was mainly
from somatosensory areas. In primates, somatosensory areas include areas that
contribute projections to subcortical motor centers.

Cortical motor and premotor areas apparently
arose with the emergence of placental mammals, and they have been modified,
expanded, and multiplied in their various branches. Although cortical sensory
areas also have motor functions, they and motor areas are organized
differently. Sensory maps are “topographic,” preserving the relationship among
sensory receptors, while motor maps are “fractured” or “mosaic,” closely
grouping neurons that contribute to cooperative movements of body parts. This
more advanced organization in placental mammals appears to provide advantages
in mediating and guiding motor behavior.

Specializations of motor and premotor cortical
areas add to human motor abilities. Perhaps the most notable ability that humans
have as a result of motor and premotor cortical specialization is a high level
of conscious control over our vocal apparatus, which allows us to speak
fluently. Another innovation has been a specialized part of the primary motor
cortex deep in the central sulcus (the brain fissure that separates the
parietal lobe from the frontal lobe and the primary motor cortex from the
primary somatosensory cortex) that provides motor control of individual
fingers.

Most primates have this capacity to a limited
extent. But the skillful finger movements needed to play the piano, type, or
use tools depend on a part of the motor cortex that sends more projections
directly to synapse on the motor neurons that activate muscles that move the
digits—a distinctively human arrangement. While macaque monkeys have such
direct projections from a "new" part of the primary motor cortex,19 this direct projection is likely proportionately and absolutely greater in
humans. Another way that motor and premotor cortex areas have been modified and
expanded in humans concerns the subdivision of these areas into patches of
neurons or “domains” for specific, functionally relevant behaviors.

The Dorsal Stream Action System

All mammals have two cortical visual systems,
a dorsal one to guide actions and a ventral one for object identification. This
concept stems from studies in the 1960s20 from which it
became apparent that the two visual systems involve complementary paths from
the retina through different brain areas.

In many mammals, the two systems function
somewhat independently, allowing these animals to retain many visual abilities even
after primary visual cortex lesions. In primates and especially in humans,
however, the ventral object identification pathway to the inferior temporal
lobe has been greatly enhanced and is totally dependent on the primary visual
cortex for information. This enhancement allows us to recognize thousands of
faces, an ability that promotes complex social systems.

The dorsal system provides visual inputs to
the posterior parietal cortex to help in the selection and guidance of
functionally relevant actions, but in primates the source of the visual
information likewise comes predominantly from connections with the primary
visual cortex. Thus, a lesion of the primary visual cortex, especially in
humans, renders both systems sub-functional, and the visual abilities that remain may be
no more than “blindsight” (the ability of people
who have visual cortex lesions to respond to visual stimuli that they do not
consciously see).

The adaptive reason for this change in
primates is not clear, but one might speculate that the growing role of the
dorsal stream to the posterior parietal cortex in visually guided behavior benefited
greatly from the more extensive information available from the primary visual
cortex compared to information in the other path that remains important in
other mammals. In humans and many other primates, 80 percent of the projections
of the retina, those concerned with producing detailed images, colors, and
aspects of contrast, go to the primary visual cortex where information is
distributed to both dorsal and ventral streams of cortical processing.

The newly enhanced dorsal stream projections
to the posterior parietal cortex were associated with a great expansion of the
posterior parietal cortex (PPC) in early primates. In these animals, the caudal
part of the PPC was devoted to further visual information processing, while
most of the rostral part was used to promote specific behaviors, based mainly
on visual, somatosensory, and (to a lesser extent), auditory information. This
cortex has been explored with electrical stimulation of the rostral part of PPC
in galagos, small primitive primates from Africa. In these animals, and as
found more recently in monkeys, stimulation of each of roughly eight subregions
produces a different complex movement.

From lateral to medial in rostral PPC, we found
subregions (or “domains”) for looking (eye movements), producing an aggressive
face, protecting the face, grasping, bringing the hand to the mouth, reaching,
and locomotion.21 These domains project to matching domains
in the premotor (PMC) and primary motor (M1) cortex, where electrical
stimulation produces the same movements. The functional reasons for three
cortical stages is not clear, but we speculate that the PPC domains are driven
by sensory inputs and that they compete with each other to relay the dominant
outcome based on sensory information to PMC and M1.

We suggest that the PMC domains re-evaluatethe processing outcomes from
PPC with
the advantage of additional information, from the prefrontal cortex, and they
activate domains in the primary motor cortex accordingly. M1 domains use
additional information from cingulate and supplementary motor areas and the
motor thalamus to interact and produce the most appropriate action.
Deactivating M1 domains renders PMC and PPC domain stimulation ineffective,
providing evidence for the hierarchy of decision-making proposed above.

This dorsal stream/action system appears to
have been more expanded and modified in more advanced primates. First, the
arrangement of domains was rotated from a lateral-to-medial to a
rostral-to-caudal cortical sequence. Second, in the evolution of modern humans,
more steps were added to the processing of visual information in the caudal
PPC, and more domains added to the rostral PPC, including one for speech
production and one for tool use. Likewise, the domain systems of the PMC and M1
likely have been expanded.

We emphasize the importance of this multistage
expandable system because the ability to decide rapidly but carefully on the
best of several alternative behaviors is especially important for humans, as
their long developmental times and delayed reproduction require a long life. Given
the limits of available information, some of the conclusions summarized above
are speculative and open to further evaluation. Fortunately, modern
neuroscience methods provide the means for a better understanding of how brains
are similar and different across species, and why that matters. For now, the
neural mechanisms mediating the astonishing abilities of the human mind remain
incompletely understood, and this should motivate us to look further. It is an
exciting time.